Although originally termed histone acetyltransferases (HATs) for their lysine acetylation activity on histone N-terminal tails, p300 and its paralogue, CBP, have been shown to acetylate a variety of non-histone proteins including p53, DNA polymerase β and nuclear import factors. p300/CBP acetylations play regulatory roles in transcription, DNA repair and replication, the cell cycle, p53 turnover, and nuclear import. In addition to functions which overlap with those of CBP, p300 has a number of unique functions, including a role in the induction of p21Waf1/Cip1 cell cycle inhibitor.

≥250 pmol/min/µg assayed as production of CoA-SH from AcCoA in the presence of a peptide comprising human p53 residues 368-386. CoA-SH is determined colorimetrically by reaction with DTNB (5,5′-Dithiobis(2-nitrobenzoic acid)).